1. Field of the Invention
The present invention relates to a dip type surface treatment apparatus of full dip or half dip, which is provided in a coating line for vehicle bodies or vehicle parts and a dip type surface treatment method, especially an electrodeposition coating apparatus and an electrodeposition coating method, and more particularly, it relates to a dip type surface treatment apparatus and a dip type surface treatment method of a countercurrent type obtained by slightly altering existing facilities.
2. Description of the Related Art
A three-coating system comprising base coating, intermediate coating and finish coating is employed for the coating of a vehicle body, but among them, for the base coating process, there is widely used, for example, a dipping coating method by which a vehicle body is fully dipped into a treatment liquid or a coating liquid in a process such as a degreasing treatment, a chemical treatment or electrodeposition coating.
In this kind of dipping coating method, since the vehicle body which is continuously carried must be fully dipped for a predetermined period of time, a large amount of the treatment liquid or the coating liquid is accommodated in a treatment tank or a coating tank.
Among others, since the electrodeposition coating liquid is diluted to a low solid content, pigment sedimentation occurs if constant or intermittent stirring is not performed. Further, a large amount of the liquid is accommodated in a tank, and hence, if the pigment once settles down, the re-dispersion of the pigment is very difficult. If the dispersion of the pigment is uneven in the electrodeposition coating liquid, a coating film becomes scabrous, which influences a finish coating film.
Furthermore, in the electrodeposition coating for forming a coating film by a cataphoresis behavior, a reaction gas may be generated on a coating surface at the time of forming the coating film, i.e., dipping. If the air bubbles are left as they stand, they remain in the coating film which is being deposited, thereby leading to a defect in the coating film. In this regard, an appropriate flow velocity must be given to the electrodeposition coating liquid in the tank to thereby remove the reaction gas from the coating surface.
Moreover, in the electrodeposition coating, since heat of reaction is produced in formation of a coating film, a coating temperature is increased in the vicinity of the coating surface and the coating resistance is lowered. When these changes are left as they are, a locally thick film may be generated. If the electrodeposition film thickness is uneven, the coating film surface quality such as visual effects or a paintwork becomes also uneven. When the film is too thick, a cost problem can also occur. Therefore, the stirring in a tank is required in order to supply the coating liquid having an appropriate temperature to the coating surface to be cooled down.
On the other hand, in a welding process which is a pre-process of the coating process, since vehicle body panels are joined to be assembled by, e.g., spot welding or arc welding, the vehicle body on which metal powder such as spatters generated at spot welding remains is carried into a coating process. In the pre-treatment process of the electrodeposition coating process, a multi-stage cleaning process is provided in order to flush away these extraneous substances, but fine metal powder or extraneous substances which have adhered in a car interior can not be completely washed away.
When such metal powder is brought into the electrodeposition tank, it again adheres to a horizontal part of the vehicle body in particular and it enters the electrodeposition coating film to provoke a defect of the coating film. Therefore, the stirring in a tank is used in order to remove extraneous substances such as metal powder which may adhere to the coating surface and discharge them to outside of a tank by a filter.
As described above, in the light of prevention of the pigment sedimentation or uniformization of the pigment dispersion, removal of air bubbles or heat, and prevention of adhesion of extraneous substances, the stirring in the electrodeposition tank is carried out.
As this type of stirring in a tank, there is known a coating circulation system disclosed in Japanese patent applications laid-open Nos. 272091-1994, 272092-1994, and 41687-1996.
The stirring in a tank according to the conventional coating circulation system is generally as follows.
That is, a vehicle body as a coating target is hanged by a hanger and brought into an electrodeposition tank, in which a electrodeposition coating liquid is filled, by an overhead conveyer at a constant speed.
The vehicle body is put into the electrodeposition tank at an angle of approximately 30xc2x0 and passes through inside the tank while assuring a full-dip time not less than three minutes. The vehicle body is then raised at angle of approximately 30xc2x0. During this process, a direct-current voltage of approximately 300 V is applied to the cationic electrodeposition coating in the electrodeposition tank through electrode plates arranged on a side wall and a bottom wall of the electrodeposition tank. As a result, cataphoresis of coating particles is generated between the electrode plates and the body as a ground side, thereby forming an electrodeposition coating film on inside and outside plates or an inner surface of a bag-like structure of the body B.
In the conventional stirring in the tank, the coating liquid is sucked from the overflow tank and then discharged into the tank, or the coating liquid is sucked from the electrodeposition tank itself and then discharged into the tank again to form a flow whose direction is equal to a moving direction of the body in a surface layer portion of the coating liquid in the tank. On the other hand, in a low layer portion of the coating liquid, a flow in an opposite direction is formed, thereby generating the large circulation of the coating liquid in the tank as a whole.
However, when a direction of the surface flow is equal to a moving direction of the body as in the conventional electrodeposition coating apparatus, there extremely increases such a tendency as that foreign particles brought by the body spread in the entire electrodeposition tank at an inlet portion of the tank. Further, since foreign particles float with movement of the body before reaching the overflow tank, such foreign particles are disadvantageously embedded in the coating film during the formation of the electrodeposition film.
When a direction of the surface flow is equal to a moving direction of the body, the relative velocity of the liquid flow and the body must be set equal to or above a predetermined value in order to maintain the effect for removing the above-described air bubbles or heat of reaction, which may increase the energy required for the stirring. In addition, there is also a problem that the extremely high liquid flow in the electrodeposition tank easily causes bubbles to be sucked and the bubbles adhere to the body. Thus, there is an upper limit for increase in the liquid flow. In the usual coating line of the vehicle body, a conveyer speed is 0.1 m/s and a liquid flow speed is 0.2 m/s, which results in the relative velocity of 0.1 m/s.
Accordingly, one of the present applicants has precedently proposed that a direction of the liquid flow in the electrodeposition tank is opposite to a moving direction of the body and the overflow tank is arranged on the inlet side of the electrodeposition tank (see Japanese patent application laid-open No. 200092-1999). With this arrangement, the relative velocity of the liquid flow and the body is increased and the effect for removing air bubbles or heat of reaction is enhanced. Further, foreign particles brought by the body can be discharged from the inlet portion directly to the overflow tank.
However, in the case that this electrodeposition coating apparatus is used, there is no problem in the installation of a new facility, but a large amount of facility cost is required when the conventional following current type electrodeposition coating apparatus is changed to the counter current type apparatus, which is not practical.
In view of the above-described problems in the prior art, an object of the present invention is to provide a dip type surface treatment apparatus and a dip type surface treatment method of the countercurrent type obtained only by slightly altering an existing facility.
To achieve this aim, there is provided a dip type surface treatment apparatus for dipping a treatment object into a treatment liquid filled in a treatment tank, the apparatus comprising: a first circulation system for causing a flow direction of the treatment liquid in a surface area and an intermediate area through which the treatment object passes in the treatment tank to be opposite to a moving direction of the treatment object; and a second circulation system for causing a flow direction of the treatment liquid in a bottom area in the treatment tank to be equal to the moving direction of the treatment object.
Further, to achieve the above aim, there is provided a dip type surface treatment method for dipping an object to be treated into a treatment liquid filled in a treatment tank, the method comprising: causing a flow direction of the treatment liquid in a surface area and an intermediate area through which the treatment object passes in the treatment tank to be opposite to a moving direction of the treatment object; and causing a flow direction of the treatment liquid in a bottom area in the treatment tank to be equal to the moving direction of the treatment object, thereby treating the treatment object.
Moreover, to achieve the above aim, there is provided an electrodeposition coating apparatus for applying a voltage between a coating object dipped in a coating liquid in an electrodeposition tank and an electrode to form a coating film, the apparatus comprising: a first circulation system for causing a flow direction of the coating liquid in a surface area and an intermediate area through which the coating object passes in the electrodeposition tank is opposite to a moving direction of the coating object; and a second circulation system for causing a flow direction of the coating liquid in a bottom area in the electrodeposition tank is equal to the moving direction of the coating object.